In a cellular mobile communication system, when a terminal, e.g., a User Equipment (UE) moves from one cell to another, the terminal needs to be handed off from the cell it originally resided in to the cell it newly enters. Through the handoff, the location of the terminal and the resource the terminal uses are updated.
Handoff is the most important part in mobility management. It refers to a process in which a link between an original sector and the terminal is replaced by a link between a new sector and the terminal when the terminal approaches the edge of an original serving cell and is to enter another serving cell.
In a Wideband Code Division Multiple Access (WCDMA) network, for example, the handoff during a call may be categorized into hard handoff, soft handoff (handoff between cells in different Node Bs) and softer handoff (handoff between sectors in the same Node B), depending on the connection between the terminal and the network, release of the connection and frequency utilization.
Soft Handoff (SHO) means that the terminal does not disconnect from an original Node B when the terminal needs to communicate with a new Node B. In other words, during a handoff between the coverage edges of two or more Node Bs, the terminal first establishes a connection with a new cell by using a same frequency before disconnecting with the original cell, and receives signals from several Node Bs at the same time until certain conditions are met. During the handoff, the terminal communicates with all candidate Node Bs at the same time over a traffic channel (TCH).
The advantage of soft handoff is that it keeps communications uninterrupted when the terminal is crossing a cell edge, which greatly reduces call drops caused by handoff. In addition, soft handoff enables a selective combination of uplink signals and a maximal combination of downlink signals, which further improves the performance of radio links.
Soft handoff in the WCDMA network involves a pilot channel and a cell set. The WCDMA system uses a pilot channel to access and handoff to a channel. The terminal identifies the strongest signal by processing the pilot channel.
The WCDMA system uses the m sequence (short PN code) to modulate the pilot channel. The time offsets of the PN code in different pilots are different, and the offset between two neighboring pilots is 64 chips. The terminal differentiates cells by identifying the offset.
There are three types of cell sets in the WCDMA system: active set, monitor set and detect set.
1. Active set: a collection of cells corresponding to the traffic channels to which the terminal is currently connected.
2. Monitored set: the cells in a monitor set do not belong to the active set but are configured as neighboring cells to the cells in the active set and are sent to the terminal by included in a measurement control message.
3. Detected set: the cells in a detected set neither belong to the active set nor being configured as adjacent cells of the cells in the active set. These cells are not included in a measurement control message.
During a handoff, the terminal often evaluates handoff events in terms of quality.
During the evaluation, a cell individual offset (CIO) is added as an offset value to a cell measurement quality so that the evaluation of handoff event can be more accurate. For each cell in a monitored set, there can be set a cell individual offset. This offset can be either positive or negative. It is add as an offset value to the cell measurement quality when the terminal performs a handoff event evaluation.
As shown in FIG. 1, the dotted curve N1 denotes a threshold for triggering event 1A, the curve N2 indicates the actual signal quality of the primary common pilot channel (PCPICH) of a cell and the curve N3 denotes the threshold after a cell individual offset is added to the actual signal quality of the PCPICH of the cell.
It can be seen from FIG. 1 that, if no cell individual offset is configured, event 1A is triggered at point B instead of at point A. In general, a telecom operator detects a sharp rise of signal quality of a cell in advance by, e.g., a drive test and configures a positive cell individual offset so that event 1A can be triggered in advance and that a cell is added to an active set in time.
In some special scenarios in an actual network, for example, at a street corner or an overhead viaduct, the signal strength of a cell may change suddenly. Therefore, to timely trigger a handoff or avoid unnecessary handoff, it is very important to correctly configure the cell individual offset.
In the prior art, the cell individual offset is configured in allusion to a cell per se. Each cell has one unique cell individual offset. Once the cell individual offset for a cell is configured, it will not change. This unique offset, however, can result in such a problem that the unique cell individual offset for a cell does not apply to all adjacent cells of this cell. For example, assume that cells 1, 2 and 3 are intra-frequency neighboring cells each other, the cell individual offset for cell 1 is set to 3 dB which may intend to optimize the handoff from cell 2 to cell 1 in, e.g., a corner scenario, so that the handoff can be triggered timely. Regarding the handoff from cell 3 to cell 2, however, there is no such a scenario of corner and thus, it is not required to use a cell individual offset for triggering the handoff timely.
In addition, when a cell is added to or removed from an active set, the information of intra-frequency neighboring cells or inter-system neighboring cells needs to be modified accordingly. In this case, the network needs to re-send an updated measurement control message including the added and/or removed neighboring cells, to a terminal. Because the cell individual offset is configured in allusion to a cell itself and is unique, the information of those unchanged cells and the cell individual offset for them do not change even when adjacent cells are added or removed. The unchanged unique cell individual offset for these unchanged cells, however, may be unsuitable for the added adjacent cells.
During the implementation of the present invention, the inventor realized at least the following problems in the prior art: the unique cell individual offset configured for a cell per ser may do not apply to all adjacent cells of the cell. Furthermore, the unchanged cell individual offset cannot reflect the actual change of a cell. Therefore, a handoff event evaluation is not accurate enough in the prior art.